Luling Wang

2-D Array Photonic Crystal Sensing Motif

We have developed the first high-diffraction-efficiency two-dimensional (2-D) photonic crystals for molecular recognition and chemical sensing applications. We prepared close-packed 2-D polystyrene particle arrays by self-assembly of spreading particle monolayers on mercury surfaces. The 2-D particle arrays amazingly diffract 80% of the incident light. When a 2-D array was transferred onto a hydrogel thin film showing a hydrogel volume change in response to a specific analyte, the array spacing was altered, shifting the 2-D array diffraction wavelength. These 2-D array photonic crystals exhibit ultrahigh diffraction efficiencies that enable them to be used for visual determination of analyte concentrations.

UV resonance Raman determination of molecular mechanism of poly(N-isopropylacrylamide) volume phase transition.

Poly(N-isopropylacrylamide) (PNIPAM) is the premier example of a macromolecule that undergoes a hydrophobic collapse when heated above its lower critical solution temperature (LCST). Here we utilize dynamic light scattering, H-NMR, and steady-state and time-resolved UVRR measurements to determine the molecular mechanism of PNIPAMs hydrophobic collapse. Our steady-state results indicate that in the collapsed state the amide bonds of PNIPAM do not engage in interamide hydrogen bonding, but are hydrogen bonded to water molecules. At low temperatures, the amide bonds of PNIPAM are predominantly fully water hydrogen bonded, whereas, in the collapsed state one of the two normal CO hydrogen bonds is lost. The NH-water hydrogen bonding, however, remains unperturbed by the PNIPAM collapse. Our kinetic results indicate a monoexponential collapse with tau approximately 360 (+/-85) ns. The collapse rate indicates a persistence length of n approximately 10. At lengths shorter than the persistence length the polymer acts as an elastic rod, whereas at lengths longer than the persistence length the polymer backbone conformation forms a random coil. On the basis of these results, we propose the following mechanism for the PNIPAM volume phase transition. At low temperatures PNIPAM adopts an extended, water-exposed conformation that is stabilized by favorable NIPAM-water solvation shell interactions which stabilize large clusters of water molecules. As the temperature increases an increasing entropic penalty occurs for the water molecules situated at the surface of the hydrophobic isopropyl groups. A cooperative transition occurs where hydrophobic collapse minimizes the exposed hydrophobic surface area. The polymer structural change forces the amide carbonyl and N-H to invaginate and the water clusters cease to be stabilized and are expelled. In this compact state, PNIPAM forms small hydrophobic nanopockets where the (i, i + 3) isopropyl groups make hydrophobic contacts. A persistent length of n approximately 10 suggests a cooperative collapse where hydrophobic interactions between adjacent hydrophobic pockets stabilize the collapsed PNIPAM.

Fabrication of Large‐Area Two‐Dimensional Colloidal Crystals

Nanoparticle coating: A suspension of colloidal particles in a water/propanol solution was layered onto a water surface, where the particles self-assembled into ordered two-dimensional hexagonal crystal arrays (>280 cm 2) within two minutes. These arrays were transferred from the water surface to other substrates (see picture) and embedded in a chitosan hydrogel for visual detection of the pH value. Copyright

Reflectivity enhanced two-dimensional dielectric particle array monolayer diffraction

Very high diffraction efficiencies (>80%) were observed from two-dimensional (2-D) photonic crystals made of monolayers of ∼490 nm diameter dielectric polystyrene spheres arranged in a 2-D hexagonal lattice on top of a liquid mercury surface. These almost close packed 2-D polystyrene particle arrays were prepared by a self-assembly spreading method that utilizes solvent evaporation from the mercury surface. Two-dimensional arrays transferred onto a dielectric glass substrate placed on top of metal mirrors show diffraction efficiencies of over 30%, which is 6- to 8-fold larger than those of the same 2-D monolayers in the absence of mirrors. A simple single particle scattering model with refraction explains the high diffraction efficiencies in terms of reflection of the high intensity forward diffraction.

Deep-Ultraviolet Resonance Raman Excitation Profiles of NH4NO3, PETN, TNT, HMX, and RDX

We measured the dispersion of the absolute-differential Raman crosssections of ammonium nitrate (NH4NO3), pentaerythritol tetranitrate (PETN), trinitrotoluene (TNT), nitroamine (HMX), and cyclotrimethylene-trinitramine (RDX) in acetonitrile and water solutions between 204 and 257 nm. The ultraviolet (UV) resonance Raman/differential Raman cross-sections of NH4NO3, PETN, TNT, HMX, and RDX dramatically increase as the excitation wavelength decreases deep into the UV to 204 nm. NH4NO3, PETN, and RDX are best resonance-enhanced by the 204 nm excitation used here, while the optimum excitation wavelength for TNT and HMX is ~230 nm. The excitation profile of TNT roughly follows its absorption band shape. The excitation profiles for the different Raman bands of each explosive molecule differ, indicating that multiple-excitation wavelength spectra are not redundant and can offer additional information on the species present. We see no evidence of any nonlinear spectral response or sample degradation at the fluences and spectral accumulation times used here. However, we previously observed such phenomena at longer spectral accumulation times and higher fluences. These results are promising for the development of standoff deep-UV Raman methods for explosive molecule determinations.

Solid state and solution nitrate photochemistry: photochemical evolution of the solid state lattice.

We examined the deep UV 229 nm photochemistry of NaNO(3) in solution and in the solid state. In aqueous solution excitation within the deep UV NO(3)¯ strong π → π* transition causes the photochemical reaction NO(3)¯ → NO(2)¯ + O·. We used UV resonance Raman spectroscopy to examine the photon dose dependence of the NO(2)¯ band intensities and measure a photochemical quantum yield of 0.04 at pH 6.5. We also examined the response of solid NaNO(3) samples to 229 nm excitation and also observe formation of NO(2)¯. The quantum yield is much smaller at ∼10(-8). The solid state NaNO(3) photochemistry phenomena appear complex by showing a significant dependence on the UV excitation flux and dose. At low flux/dose conditions NO(2)¯ resonance Raman bands appear, accompanied by perturbed NO(3)¯ bands, indicating stress in the NaNO(3) lattice. Higher flux/dose conditions show less lattice perturbation but SEM shows surface eruptions that alleviate the stress induced by the photochemistry. Higher flux/dose measurements cause cratering and destruction of the NaNO(3) surface as the surface layers are converted to NO(2)¯. Modest laser excitation UV beams excavate surface layers in the solid NaNO(3) samples. At the lowest incident fluxes a pressure buildup competes with effusion to reach a steady state giving rise to perturbed NO(3)¯ bands. Increased fluxes result in pressures that cause the sample to erupt, relieving the pressure.

Periodicity-Controlled Two-Dimensional Crystalline Colloidal Arrays.

We developed a convenient and fast approach to preparing close-packed two-dimensional (2-D) particle arrays on mercury surfaces. Addition of cosolvents, such as alcohols, to aqueous colloidal particle suspensions induces spreading and self-assembly of the particles into 2-D arrays on top of the mercury surface. We can fabricate large-area close-packed 2-D arrays (>70 cm(2)) within 30 s. We attached these 2-D arrays to functional hydrogel films such that the 2-D array spacings were altered by the hydrogel volume response to the environment. We directly observed the hydrogel volume induced 2-D array spacing changes by using confocal laser scanning microscopy to monitor the spacings of fluorescent polystyrene particle 2-D arrays in response to changes in pH, solvent composition, temperature, etc.

Vertical spreading of two-dimensional crystalline colloidal arrays

We report on a novel self-driven climbing of two-dimensional (2-D) ordered monolayer crystalline colloidal arrays (CCAs). This phenomenon can be used to rapidly and efficiently prepare large area, highly ordered 2-D array monolayer CCA films on various substrates. Large area 2-D polystyrene (PS) particle CCAs were fabricated on water surfaces by a needle tip flow technique. Introduction of a wet substrate through the 2-D particle monolayer array on the water surface causes the 2-D array to flow onto the wet substrate surface due to a surface spreading pressure. This method can quickly prepare ordered 2-D particle arrays on numerous wet substrates including flat/curved glass slides, inner walls of glass tubes, hydrogels, flexible polymer films, patterned surfaces, etc. By using responsive hydrogels as substrates, we can conveniently prepare 2-D photonic crystal sensors that can be used to visually determine analyte concentrations. For example, we prepared 580 nm PS 2-D arrays on poly(2-hydroxyethyl methacrylate) hydrogels to sense ethanol in water.

Refractive-Index Matching Avoids Local Field Corrections and Scattering Bias in Solid-State Na2SO4 Ultraviolet Raman Cross-Section Measurements.

We report a refractive-index matching method to measure nonabsorbing solid ultraviolet (UV) Raman cross-sections that avoids the local field correction and interface scattering of incident light. We used refractiveindex-matched chloroform as an internal standard to determine the solid-state 995 cm−1 Na2SO4 244 nm Raman cross-sections. The pure liquid chloroform 668 cm−1 244 nm Raman cross-section was determined by using acetonitrile as an internal standard and by calculating the local field corrections for the observed Raman intensities. Our measured 244 nm UV Raman cross-section of the solid-state 995 cm−1 SO42– band of 1.97 ± 0.07 × 10−28 cm2/(molc·sr) is about half of its aqueous solution Raman cross-section, indicating interactions between the sulfate species in the solid that decrease the Raman polarizability.

Silica Crystalline Colloidal Array Deep Ultraviolet Narrow-Band Diffraction Devices

We developed a facile method to fabricate deep ultraviolet (UV) photonic crystal crystalline colloidal array (CCA) Bragg diffraction devices. The CCAs were prepared through the self-assembly of small, monodisperse, highly surface charged silica particles (~50 nm diameter) that were synthesized by using a modified Stöber process. The particle surfaces were charged by functionalizing them with the strong acid, non-UV absorbing silane coupling agent 3-(trihydroxylsilyl)-1-propane-sulfonic acid (THOPS). These highly charged, monodisperse silica particles self assemble into a face-centered cubic CCA that efficiently Bragg diffracts light in the deep UV. The diffracted wavelength was varied between 237 nm to 227 nm by tilting the CCA orientation relative to the incident beam between glancing angles from 90° to ~66°. Theoretical calculations predict that the silica CCA diffraction will have a full width at half-maximum (FWHM) of 2 nm with a transmission of ~10−11 at the band center. We demonstrate the utility of this silica CCA filter to reject the Rayleigh scattering in 229 nm deep UV Raman measurements of highly scattering Teflon.